1. Field of the Invention
The present invention relates to a tyre for vehicle wheels and to a method of manufacturing it.
2. Description of the Related Art
A tyre for vehicle wheels usually comprises a carcass structure essentially consisting of one or more carcass plies shaped in a substantially toroidal conformation and having their axially opposite side edges in engagement with respective annular reinforcing structures, each comprising at least one bead core, each ply having a plurality of cords parallel to each other incorporated into at least one layer of elastomer material. Each annular reinforcing structure is incorporated in a so-called “bead” defined along an inner circumferential edge of the tyre for tyre anchoring to a corresponding mounting rim.
In the so-called “radial” tyres each of the cords arranged in the carcass ply or plies lies in a plane substantially radial to the tyre rotation axis, i.e. each cord has an orientation substantially orthogonal to the circumferential extension direction.
In this case a belt structure is generally applied to the carcass structure, at a radially external position, which belt structure comprises one or more belt strips shaped like a closed ring and essentially consisting of textile or metallic cords incorporated in a layer of elastomer material, parallel to each other and suitably oriented relative to the cords belonging to the adjacent carcass plies and/or belt strips.
Coaxially arranged at a radially external position to the belt structure is a tread band intended for rolling contact of the tyre on the ground and usually provided with a raised pattern defined by cuts and grooves formed in the band thickness which are adapted to ensure the necessary grip qualities of the tyre on the ground.
It is to be pointed out, to the aims of the present invention, that by the term “elastomer material” it is intended a composition comprising at least one elastomer polymer and at least one reinforcing filler and possibly additives of different types (cross-linking agents, plasticizers and other additives known in the art, for example).
Provided on the opposite sides of the tyre is a pair of sidewalls each of which covers a side portion of the tyre included between a so-called shoulder region located close to the corresponding side edge of the tread band, and the corresponding bead.
In the traditional manufacturing processes, formation of each carcass ply is carried out by making a rubberised fabric comprising a plurality of reinforcing thread-like elements, incorporated in a layer of elastomer material, parallel to each other, in the following referred to as “cords”; this term is used to define both individual strands (monofilaments, flosses, yarns, wires) and true cords, i.e. a structure comprising one or more strands twisted together, each consisting of a given number of elementary filaments twisted with each other. Said rubberised fabric is made through a calendering operation for example, in which a plurality of said cords disposed in coplanar side by side relationship with each other are sandwiched between two layers of elastomer material. The threads of said cords may consist of metal alloys such as steel, textile fibres, carbon fibres, glass fibres, Kevlar® or other fibres having an appropriate resistance for use in making tyres.
Each carcass ply is then assembled with the other tyre components, such as said bead cores for example, on a so-called cylindrical building drum. The carcass ply is turned up around said bead cores in such a manner that when the whole carcass structure, first made in the form of a cylindrical sleeve, is shaped into a toroidal conformation, the carcass ply and bead cores keep associated with each other, the carcass ply being turned up around said bead cores and extending from the inside to the outside both axially and radially.
In a subsequent operation other tyre components, such as belt structure, sidewalls and tread band, are then assembled (on a shaping drum for example) on said carcass structure.
According to recent production processes, as depicted in document EP 928 680 for example, in the name of the same Applicant, a tyre can be directly built on a toroidal support. A first carcass ply is formed by laying strip-like elements on said toroidal support, each of said strip-like elements comprising longitudinal thread-like elements incorporated in a layer of elastomer material. Said strip-like elements are sequentially laid down so as to form a carcass structure in which the strip-like elements are partly overlapped at side portions of the tyre and disposed in circumferential side by side relationship with each other at the crown region of the tyre itself. Annular reinforcing structures are associated with the carcass ply which comprise a first and a second circumferential annular inserts for example, and an elastomer filler interposed therebetween. A second carcass ply can be made in superposed position with respect to the first carcass ply and said annular structures. Then a belt structure also made by deposition of strip-like elements is associated with the carcass structure thus formed. Subsequently, a tread band and a pair of sidewalls are applied by superposition of an elementary semi-finished product of elastomer material in the form of a strap of appropriate sizes laid down into coils disposed in axial side by side and/or radial superposition relationship. Therefore, this process contemplates use of at least two different types of elementary semi-finished products and more specifically a strap, i.e. a section member made of elastomer material alone of substantially rectangular section and the strip-like element of elastomer material into which thread-like reinforcing elements are incorporated, typically textile or metallic cords. The strip-like element thus made contains a given number of cords per centimeter of its transverse dimension, and said number identifies the “density” of the cords.
It should be noticed that the tyre made in accordance with the above description has its carcass structure associated with the annular reinforcing structures but not turned up with respect to the latter. To comply with the requirements of a carcass structure more linked to the annular reinforcing structures and obviate possible structural problems arising from the carcass ply partly slipping off the annular reinforcing structures during tyre manufacture, some solutions have been proposed.
For example, document EP 0 983 876 discloses a radial tyre in which the carcass ply is formed of a continuous cord repeating in succession a path between a pair of bead portions in a direction circumferential to the tyre.
Document WO 00/26043 in the name of the same Applicant discloses a tyre and a method of making it, in which the carcass structure comprises at least one carcass ply formed of strip-like elements comprising longitudinal thread-like elements incorporated in a layer of elastomer material. The strip-like elements have respective crown portions disposed in circumferential side by side relationship with each other, as well as side portions radially extending at the tyre sidewalls. The side portions are each at least partly covered with a side portion belonging to a stretch of an adjacent portion. Associated with the carcass ply are annular structures each of which comprises a pair of inextensible inserts having the conformation of an annulus, between which a turned-up flap of the carcass ply is interposed, after possible interposition of an elastomer filling insert.
The Applicant has ascertained that a tyre built in accordance with the above mentioned documents can have some drawbacks. In particular, the Applicant could notice that the diameter in the crown region of the toroidal support on which building takes place, is bigger than the diameter at the beads and this condition is more marked in the toroidal supports designed for making tyres for heavy-duty vehicles such as trucks, articulated lorries and the like. Consequently, deposition of a plurality of elongated elements circumferentially adjacent to each other in the crown region of the carcass causes overlapping of the ends of said elements in the bead region. This circumstance brings to construction of beads having an apparently reinforced structure due to the increased density of the cords, which structure is on the contrary substantially weakened. In fact, under use conditions the mutually superposed cords do not work correctly and in a uniform manner, some of them being strongly submitted to tensile stresses, other to compressive stresses, thereby triggering separation between the elastomer material and the reinforcing cords so that, as a result, the tyre will have a shorter duration.
The Applicant has also ascertained that turning up such a structure, i.e. a carcass ply obtained by approaching and/or partially overlapping a plurality of elongated elements around an annular reinforcing structure, generates a further increase in the structural unevenness in a critical tyre region such as the bead region, which unevenness too has then repercussions on the tyre duration as it generates irregularities in stress distribution during rolling and possible fatigue failure in relatively short periods of time.